While monotheists may believe in the existence of lesser supernatural beings such as angels, and may even accept the existence of entities that are worshiped as gods by others, they will not consider them to be gods. (I'm specifically thinking of C.S. Lewis's Space Trilogy, in which he portrays the members of the Greco-Roman pantheon as angels mistaken for gods by ancient peoples.) The difference is most discernable in the question of worship - Christians and Muslims may believe in angels, but they would consider it a grievous sin to worship one since in their theology only God is worthy of worship.
I'm not qualified to comment on the issue of whether or not Roman Catholics worship saints, so I won't. Maybe some Catholic Everythingian could chime in and explain how they figure in the practice of their faith?